Polarization scramblers are devices that converts highly polarized light into light with a scrambled polarization by modulating the state of polarization("SOP"). Degree of polarization("DOP") represents the ratio of polarized light to total light. For example, the DOP of perfectly polarized light is 100%, whereas the DOP of completely depolarized light is 0%. The function of the polarization scrambler is to decrease the DOP in time average. An ideal polarization scrambler forces input light of 100% DOP to output light of 0% DOP.
When light goes through an optical component having a polarization-dependent loss, output power depends on the input SOP of the light. In this case, constant time-averaged output power can be obtained regardless of the input SOP by inserting a polarization scrambler in front of the optical component. Enhanced signal-to-noise ratio can also be obtained with the polarization scrambler in a fiber-optic sensor, optical measurement system and long-haul optical transmission system.
The output SOP is evolved by a birefringence medium, especially the amount of the birefringence and the angle of the birefringence axis when polarized input light is propagating along the optical fiber. For the light which has been transmitted along a long optical fiber over a few meters, however, the output SOP is apt to change since the birefringence of the fiber is easily affected by small environmental perturbation such as temperature and pressure. This leads to output power fluctuation of the light which went through an optical device having certain polarization-dependent loss. However, the fluctuation of the output light could be prevented if it is averaged in time by decreasing the DOP of the output light through polarization modulation.
If the polarization direction of the input light coincides with the direction of the birefringence axis of the birefringence modulator, the output SOP will not vary even with any change of the birefringence. Therefore, in order to induce polarization modulation of the output light regardless of the input SOP using birefringence amplitude modulation, at least two polarization modulators whose birefringence axes form an angle of 45.degree. must be used.
In the prior art, a polarization scrambler is implemented by inducing birefringence modulation in an integrated optical circuit, such as a lithium niobate (LiNbO.sub.3) optical waveguide. However, this kind of the polarization scrambler has the disadvantages of low efficiency, difficulty in connecting two or more birefringence modulators, and high insertion loss.
U.S. Pat. No. 4,923,290 discloses a basic concept of the polarization scrambler using an optical fiber. However, since the method of directly exerting pressure onto the optical fiber is used to implement the scrambler, the device for inducing the birefringence has a low efficiency with scrambling frequency lower than few hundred Hz, which is too low. Also the birefringence axis is subject to easy change and the output SOP deteriorates. This renders the realization of the polarization scrambler with good performance difficult.